LM2907/LM2917
Frequency to Voltage Converter
General Description
The LM2907, LM2917 series are monolithic frequency to
voltage converters with a high gain op amp/comparator designed to operate a relay, lamp, or other load when the input
frequency reaches or exceeds a selected rate. The tachometer uses a charge pump technique and offers frequency
doubling for low ripple, full input protection in two versions
(LM2907-8, LM2917-8) and its output swings to ground for a
zero frequency input.
The op amp/comparator is fully compatible with the tachometer and has a floating transistor as its output. This feature
allows either a ground or supply referred load of up to 50 mA.
The collector may be taken above VCC up to a maximum VCE
of 28V.
The two basic configurations offered include an 8-pin device
with a ground referenced tachometer input and an internal
connection between the tachometer output and the op amp
non-inverting input. This version is well suited for single
speed or frequency switching or fully buffered frequency to
voltage conversion applications.
The more versatile configurations provide differential tachometer input and uncommitted op amp inputs. With this
version the tachometer input may be floated and the op amp
becomes suitable for active filter conditioning of the tachometer output.
Both of these configurations are available with an active
shunt regulator connected across the power leads. The
regulator clamps the supply such that stable frequency to
voltage and frequency to current operations are possible
with any supply voltage and a suitable resistor.
Advantages
n Easy to use; VOUT = fIN x VCC x R1 x C1
n Only one RC network provides frequency doubling
n Zener regulator on chip allows accurate and stable
frequency to voltage or current conversion (LM2917)
Features
n Ground referenced tachometer input interfaces directly
with variable reluctance magnetic pickups
n Op amp/comparator has floating transistor output
n 50 mA sink or source to operate relays, solenoids,
meters, or LEDs
n Frequency doubling for low ripple
n Tachometer has built-in hysteresis with either differential
input or ground referenced input
n Built-in zener on LM2917
n ± 0.3% linearity typical
n Ground referenced tachometer is fully protected from
damage due to swings above VCC and below ground
Applications
n
n
n
n
n
n
n
n
n
n
n
Over/under speed sensing
Frequency to voltage conversion (tachometer)
Speedometers
Breaker point dwell meters
Hand-held tachometer
Speed governors
Cruise control
Automotive door lock control
Clutch control
Horn control
Touch or sound switches
n Output swings to ground for zero frequency input
Block and Connection Diagrams
Dual-In-Line and Small Outline Packages, Top Views
DS007942-1
DS007942-2
Order Number LM2907M-8 or LM2907N-8
See NS Package Number M08A or N08E
© 2001 National Semiconductor Corporation
DS007942
Order Number LM2917M-8 or LM2917N-8
See NS Package Number M08A or N08E
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LM2907/LM2917 Frequency to Voltage Converter
June 2000
LM2907/LM2917
Block and Connection Diagrams
Dual-In-Line and Small Outline Packages, Top Views (Continued)
DS007942-3
DS007942-4
Order Number LM2907M or LM2907N
See NS Package Number M14A or N14A
www.national.com
Order Number LM2917M or LM2917N
See NS Package Number M14A or N14A
2
Power Dissipation
LM2907-8, LM2917-8
1200 mW
LM2907-14, LM2917-14
1580 mW
See (Note 1)
Operating Temperature Range
−40˚C to +85˚C
Storage Temperature Range
−65˚C to +150˚C
Soldering Information
Dual-In-Line Package
Soldering (10 seconds)
260˚C
Small Outline Package
Vapor Phase (60 seconds)
215˚C
Infrared (15 seconds)
220˚C
See AN-450 “Surface Mounting Methods and Their Effect
on Product Reliability” for other methods of soldering
surface mount devices.
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Supply Voltage
Supply Current (Zener Options)
Collector Voltage
Differential Input Voltage
Tachometer
Op Amp/Comparator
Input Voltage Range
Tachometer
LM2907-8, LM2917-8
LM2907, LM2917
Op Amp/Comparator
28V
25 mA
28V
28V
28V
± 28V
0.0V to +28V
0.0V to +28V
Electrical Characteristics
VCC = 12 VDC, TA = 25˚C, see test circuit
Symbol
Parameter
Conditions
Min
Typ
Max
Units
± 10
± 25
± 40
mV
TACHOMETER
Input Thresholds
VIN = 250 mVp-p @ 1 kHz (Note 2)
Hysteresis
VIN = 250 mVp-p @ 1 kHz (Note 2)
Offset Voltage
VIN = 250 mVp-p @ 1 kHz (Note 2)
30
LM2907/LM2917
LM2907-8/LM2917-8
mV
3.5
10
mV
5
15
mV
1
µA
Input Bias Current
VIN = ± 50 mVDC
0.1
VOH
Pin 2
VIN = +125 mVDC (Note 3)
8.3
VOL
Pin 2
VIN = −125 mVDC (Note 3)
2.3
I2, I3
Output Current
V2 = V3 = 6.0V (Note 4)
I3
Leakage Current
I2 = 0, V3 = 0
K
Gain Constant
Linearity
V
V
140
180
240
µA
(Note 3)
0.9
1.0
0.1
1.1
µA
fIN = 1 kHz, 5 kHz, 10 kHz (Note 5)
−1.0
0.3
+1.0
%
OP/AMP COMPARATOR
VOS
VIN = 6.0V
3
10
mV
IBIAS
VIN = 6.0V
50
500
nA
VCC−1.5V
V
Input Common-Mode
Voltage
0
Voltage Gain
200
V/mV
50
mA
Output Sink Current
VC = 1.0
Output Source Current
VE = VCC −2.0
10
Saturation Voltage
ISINK = 5 mA
0.1
40
ISINK = 20 mA
ISINK = 50 mA
1.0
RDROP = 470Ω
7.56
mA
0.5
V
1.0
V
1.5
V
15
Ω
ZENER REGULATOR
Regulator Voltage
Series Resistance
10.5
Temperature Stability
+1
TOTAL SUPPLY CURRENT
3.8
V
mV/˚C
6
mA
Note 1: For operation in ambient temperatures above 25˚C, the device must be derated based on a 150˚C maximum junction temperature and a thermal resistance
of 101˚C/W junction to ambient for LM2907-8 and LM2917-8, and 79˚C/W junction to ambient for LM2907-14 and LM2917-14.
Note 2: Hysteresis is the sum +VTH − (−VTH), offset voltage is their difference. See test circuit.
Note 3: VOH is equal to 3⁄4 x VCC − 1 VBE, VOL is equal to 1⁄4 x VCC − 1 VBE therefore VOH − VOL = VCC/2. The difference, VOH − VOL, and the mirror gain, I2/I3,
are the two factors that cause the tachometer gain constant to vary from 1.0.
3
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LM2907/LM2917
Absolute Maximum Ratings (Note 1)
LM2907/LM2917
Electrical Characteristics
(Continued)
Note 4: Be sure when choosing the time constant R1 x C1 that R1 is such that the maximum anticipated output voltage at pin 3 can be reached with I3 x R1. The
maximum value for R1 is limited by the output resistance of pin 3 which is greater than 10 MΩ typically.
Note 5: Nonlinearity is defined as the deviation of VOUT (@ pin 3) for fIN = 5 kHz from a straight line defined by the VOUT @ 1 kHz and VOUT @ 10 kHz. C1 = 1000 pF,
R1 = 68k and C2 = 0.22 mFd.
Test Circuit and Waveform
DS007942-6
Tachometer Input Threshold Measurement
DS007942-7
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4
LM2907/LM2917
Typical Performance Characteristics
Total Supply Current
Zener Voltage vs
Temperature
Normalized Tachometer
Output vs Temperature
DS007942-40
DS007942-41
Normalized Tachometer
Output vs Temperature
Tachometer Currents I2
and I3 vs Supply Voltage
DS007942-43
Tachometer Linearity
vs Temperature
DS007942-42
Tachometer Currents I2
and I3 vs Temperature
DS007942-44
Tachometer Linearity
vs Temperature
DS007942-45
Tachometer Linearity vs R1
DS007942-48
DS007942-47
DS007942-46
5
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LM2907/LM2917
Typical Performance Characteristics
Tachometer Input Hysteresis
vs Temperature
(Continued)
Op Amp Output Transistor
Characteristics
DS007942-49
DS007942-50
DS007942-51
The size of C2 is dependent only on the amount of ripple
voltage allowable and the required response time.
Applications Information
The LM2907 series of tachometer circuits is designed for
minimum external part count applications and maximum versatility. In order to fully exploit its features and advantages
let’s examine its theory of operation. The first stage of operation is a differential amplifier driving a positive feedback
flip-flop circuit. The input threshold voltage is the amount of
differential input voltage at which the output of this stage
changes state. Two options (LM2907-8, LM2917-8) have
one input internally grounded so that an input signal must
swing above and below ground and exceed the input thresholds to produce an output. This is offered specifically for
magnetic variable reluctance pickups which typically provide
a single-ended ac output. This single input is also fully
protected against voltage swings to ± 28V, which are easily
attained with these types of pickups.
The differential input options (LM2907, LM2917) give the
user the option of setting his own input switching level and
still have the hysteresis around that level for excellent noise
rejection in any application. Of course in order to allow the
inputs to attain common-mode voltages above ground, input
protection is removed and neither input should be taken
outside the limits of the supply voltage being used. It is very
important that an input not go below ground without some
resistance in its lead to limit the current that will then flow in
the epi-substrate diode.
Following the input stage is the charge pump where the input
frequency is converted to a dc voltage. To do this requires
one timing capacitor, one output resistor, and an integrating
or filter capacitor. When the input stage changes state (due
to a suitable zero crossing or differential voltage on the input)
the timing capacitor is either charged or discharged linearly
between two voltages whose difference is VCC/2. Then in
one half cycle of the input frequency or a time equal to 1/2 fIN
the change in charge on the timing capacitor is equal to
VCC/2 x C1. The average amount of current pumped into or
out of the capacitor then is:
CHOOSING R1 AND C1
There are some limitations on the choice of R1 and C1 which
should be considered for optimum performance. The timing
capacitor also provides internal compensation for the charge
pump and should be kept larger than 500 pF for very accurate operation. Smaller values can cause an error current on
R1, especially at low temperatures. Several considerations
must be met when choosing R1. The output current at pin 3
is internally fixed and therefore VO/R1 must be less than or
equal to this value. If R1 is too large, it can become a
significant fraction of the output impedance at pin 3 which
degrades linearity. Also output ripple voltage must be considered and the size of C2 is affected by R1. An expression
that describes the ripple content on pin 3 for a single R1C2
combination is:
It appears R1 can be chosen independent of ripple, however
response time, or the time it takes VOUT to stabilize at a new
voltage increases as the size of C2 increases, so a compromise between ripple, response time, and linearity must be
chosen carefully.
As a final consideration, the maximum attainable input frequency is determined by VCC, C1 and I2:
USING ZENER REGULATED OPTIONS (LM2917)
For those applications where an output voltage or current
must be obtained independent of supply voltage variations,
the LM2917 is offered. The most important consideration in
choosing a dropping resistor from the unregulated supply to
the device is that the tachometer and op amp circuitry alone
require about 3 mA at the voltage level provided by the
zener. At low supply voltages there must be some current
flowing in the resistor above the 3 mA circuit current to
operate the regulator. As an example, if the raw supply
varies from 9V to 16V, a resistance of 470Ω will minimize the
zener voltage variation to 160 mV. If the resistance goes
under 400Ω or over 600Ω the zener variation quickly rises
above 200 mV for the same input variation.
The output circuit mirrors this current very accurately into the
load resistor R1, connected to ground, such that if the pulses
of current are integrated with a filter capacitor, then VO = ic x
R1, and the total conversion equation becomes:
VO = VCC x fIN x C1 x R1 x K
Where K is the gain constant — typically 1.0.
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Op Amp Output Transistor
Characteristics
6
LM2907/LM2917
Typical Applications
Minimum Component Tachometer
DS007942-8
DS007942-9
7
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LM2907/LM2917
Typical Applications
(Continued)
Zener Regulated Frequency to Voltage Converter
DS007942-10
Breaker Point Dwell Meter
DS007942-11
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8
LM2907/LM2917
Typical Applications
(Continued)
Voltage Driven Meter Indicating Engine RPM
VO = 6V @ 400 Hz or 6000 ERPM (8 Cylinder Engine)
DS007942-12
Current Driven Meter Indicating Engine RPM
IO = 10 mA @ 300 Hz or 6000 ERPM (6 Cylinder Engine)
DS007942-13
9
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LM2907/LM2917
Typical Applications
(Continued)
Capacitance Meter
VOUT = 1V–10V for CX = 0.01 to 0.1 mFd
(R = 111k)
DS007942-14
Two-Wire Remote Speed Switch
DS007942-15
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10
LM2907/LM2917
Typical Applications
(Continued)
100 Cycle Delay Switch
DS007942-16
Variable Reluctance Magnetic Pickup Buffer Circuits
DS007942-39
Precision two-shot output frequency
equals twice input frequency.
DS007942-17
Pulse height = VZENER
11
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LM2907/LM2917
Typical Applications
(Continued)
Finger Touch or Contact Switch
DS007942-19
DS007942-18
Flashing LED Indicates Overspeed
DS007942-20
Flashing begins when fIN ≥ 100 Hz.
Flash rate increases with input frequency
increase beyond trip point.
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12
LM2907/LM2917
Typical Applications
(Continued)
Frequency to Voltage Converter with 2 Pole Butterworth Filter to Reduce Ripple
DS007942-21
Overspeed Latch
DS007942-23
DS007942-22
13
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LM2907/LM2917
Typical Applications
(Continued)
Some Frequency Switch Applications May Require Hysteresis in the
Comparator Function Which can be Implemented in Several Ways:
DS007942-24
DS007942-25
DS007942-26
DS007942-27
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DS007942-28
14
LM2907/LM2917
Typical Applications
(Continued)
Changing the Output Voltage for an Input Frequency of Zero
DS007942-30
DS007942-29
Changing Tachometer Gain Curve or Clamping the Minimum Output Voltage
DS007942-32
DS007942-31
15
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LM2907/LM2917
Anti-Skid Circuit Functions
“Select-Low” Circuit
DS007942-34
VOUT is proportional to the lower of the two
input wheel speeds.
DS007942-33
“Select-High” Circuit
DS007942-36
VOUT is proportional to the higher of the two
input wheel speeds.
DS007942-35
“Select-Average” Circuit
DS007942-37
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16
17
*This connection made on LM2907-8 and LM2917-8 only.
**This connection made on LM2917 and LM2917-8 only.
DS007942-38
LM2907/LM2917
Equivalent Schematic Diagram
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LM2907/LM2917
Physical Dimensions
inches (millimeters) unless otherwise noted
8-Lead (0.150" Wide) Molded Small Outline Package, JEDEC
Order Number LM2907M-8 or LM2917M-8
NS Package Number M08A
Molded SO Package (M)
Order Number LM2907M or LM2917M
NS Package Number M14A
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18
LM2907/LM2917
Physical Dimensions
inches (millimeters) unless otherwise noted (Continued)
Molded Dual-In-Line Package (N)
Order Number LM2907N-8 or LM2917N-8
NS Package Number N08E
Molded Dual-In-Line Package (N)
Order Number LM2907N or LM2917N
NS Package Number N14A
19
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LM2907/LM2917 Frequency to Voltage Converter
Notes
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